This invention relates generally to stress testing of materials, and more particularly the invention relates to a method and apparatus for biaxially testing materials.
The strength of materials is measured by applying compressive and tensile stresses thereto. All possible biaxial stress states may be represented in a stress plane defining unit stress in X and Y coordinates with tensile stress being positive and compressive stress being negative. The first quadrant represents those stress states where both principal stresses are tensile. The second and fourth quadrants represent stress states where one stress is tensile and the other compressive. Quadrant three represents those stress states where both principal stresses are compressive. Pure shear is represented by a line bisecting quadrants two and four where unit stress in the X direction is equal and opposite to the unit stress in the Y direction.
When a material is subjected to certain combinations of stress it will fail. When all of these combinations are plotted on the biaxial principal stress plane, a biaxial failure envelope is generated. If a specimen is subjected to stress states lying inside of the failure envelope it will be undamaged. However, subjecting a specimen to stress states lying on or outside of the biaxial failure envelope will cause failure in the material.
Certain materials such as laminates and fiber reinforced composites exhibit anisotropic failure. A number of specimen load test systems have been used in the past to characterize the behavior of composite samples exhibiting anisotropic behavior under biaxial states of stress. One approach for characterizing laminates is the off-axis tensile test. With this test the material axis and the load direction are purposely not the same, thus the stress state relative to the material axis is biaxial. Thin wall tube specimens have been employed in testing of composites wherein rotation of the principal stress directions may be achieved by applying torques about the longitudinal axis of the tube. However, in composite or brittle materials premature failure may occur due to extraneous stresses imposed by changes in specimen sections or by the grips for applying torque. Other biaxial tests include the plate bulge test wherein a plate is hydraulically forced through a specially shaped opening, and the use of cruciform specimens.
All known prior art systems for biaxially testing a specimen have limitations in providing consistent and accurate test results.